US 7387123 B2
A gas delivery system, having a gas identification, by which a gas supplied to the gas delivery system is identified, a blender, blending oxygen and the gas to provide a gas mixture with an oxygen flow rate set up by an operator, and at least one flow sensor, to measure a flow rate of the gas mixture. The blender is driven by an actuator motor to various blender positions with the blender positions being calibrated based on the specific heat ratio and the gas constant of the gas. The flow sensor can be installed at the inspiratory circuit, the proximal circuit and the expiratory circuit of the gas delivery system. The flow sensor output is corrected based on the actual conditions, including the temperature, pressure and humidity, and characteristics of the gas mixture.
1. A gas delivery system, comprising:
a gas identifier, by which a gas supplied to the gas delivery system is automatically identified;
a blender, blending oxygen and the gas to provide a gas mixture with a preselected oxygen flow rate;
an actuator, driving the blender into various blending positions according to the oxygen flow rate of the gas mixture, wherein the blending positions of the blender are corrected based on characteristics of the gas mixture; and
at least one flow sensor, to measure a flow rate of the gas mixture, wherein the flow sensor is corrected based on the characteristics of the gas mixture.
2. A ventilating system, comprising:
at least two gas inlets, with one inlet connected to an oxygen source and the other one connected to a gas source;
a gas identifier, attached to the gas inlet connected to the gas source to identify a gas supplied therefrom;
a blender, blending the oxygen and the gas into a mixture, wherein the blender is driven by an actuator into various positions according to a selected oxygen flow rate of the mixture, and the blender positions are calibrated based on the specific heat ratio and gas constant of the gas;
an inspiratory circuit, on which an inspiratory flow sensor is installed, wherein the inspiratory flow sensor is calibrated according to temperature, pressure and humidity in the inspiratory circuit, and a gas constant of the gas;
a proximal circuit, through which a patient inhales and exhales, on which a proximal flow sensor is installed, wherein the proximal flow sensor is calibrated according to temperature, pressure, and humidity in the proximal circuit, and a gas constant of the gas; and
an expiratory circuit, on which an expiratory flow sensor is installed, wherein the expiratory flow sensor is calibrated according to temperature, pressure and humidity in the expiratory circuit, and a gas constant of the gas.
3. The ventilating system according to
4. The ventilating system according to
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6. The ventilating system according to
7. The ventilating system according to
The present invention relates generally to a patient ventilation system, and more particularly, to a gas identification (ID) and a volumetrically corrected gas delivery system applied to a ventilation system.
Ventilation treatments have been used as a therapeutic intervention to relieve dyspnea for a patient in acute respiratory distress directly related to severe increases in airway resistance. For moving gas into and out of the lungs, an open airway for the gas to flow from the higher pressure zone to the lower pressure zone is required. The greater the pressure between two points of the airway, the greater volume of gas is moved within the airway. The pressure in airway is directly related to the dynamic pressure gradient during the respiratory cycle, the flow rate of the gas, the density and viscosity of the gas, and the caliber and length of the airway.
Being an inert gas, helium does not participate or interfere with any biochemical process of the body. That is, helium itself has no curative value and cannot support life. However, as helium is the second lightest gas, it is often mixed with oxygen to decrease the gas density, so as to decrease the amount of pressure required for moving gas through the airway. For clinical application, helium typically is mixed with at least 21% oxygen due to its lack of pharmacological properties.
According to specific condition of each patient, the composition of the mixture of helium and oxygen (heliox) is altered. For example, when the patient is in need of a higher concentration of oxygen, a higher driving pressure is required for delivering the gas to the patient. On the contrary, when a lower concentration of oxygen is needed by the patient, the concentration of helium can be increased to dilute the oxygen, and to thereby reduce the required driving pressure of the gas. In order for the therapist/clinician to effectively provide adequate ventilation in obstructive disease patients such as chronic obstructive pulmonary disease or asthma, it is imperative that the clinician understands the relationship between airway resistance and gas characteristics.
Prior art ventilating systems that supply oxygen to a patient normally have two gas inlets, one of which is connected to an oxygen source, and the other is connected to a second gas source. Heliox with 80% of helium and 20% of oxygen, heliox with 70% of helium and 30% of oxygen, or air (with 21% of oxygen and 78% of nitrogen) are frequently applied as the second gas in such ventilating systems. After entering the ventilating system, oxygen and the second gas are mixed in a blender. According to the specific condition of the patient, the fractional inspiratory oxygen flow rate (FIO2) of the gas mixture (oxygen and the second gas) is set by the clinician, and the volumes of oxygen and the second gas entering and exiting the blender are controlled to provide an adequate oxygen flow rate (FIO2) to the patient. However, such adjustment may be inaccurate when the gas composition is altered. For example, when one applies heliox with 80% of helium and 20% of oxygen to a ventilating system of which the blender is driven according to the volumetric flow of heliox with 70% helium and 30% oxygen, the actual FIO2 air or oxygen flow rate is very likely lower than the clinician set value. In addition to obtaining an ineffective ventilation, such inaccuracy may cause a life threatening event, especially when a patient is in critical condition.
Being controlled by the blender, the gas mixture is then delivered to the patient. Again, the exact gas flow rate delivered to the patient is critical. Therefore, a flow sensor is typically installed in the prior art on the inspiratory circuit of the ventilating system to ensure that an adequate rate of gas is flowing to the patient. Similarly, the expiratory flow is also typically monitored and controlled in the prior art via a flow sensor installed in the expiratory flow circuit. To obtain an accurate measurement of the oxygen flow rate, the blender and the flow meters are calibrated before being used. However, the blender and the flow meters incorporated in the conventional prior art ventilating system are calibrated for air instead of the exact gas composition supplied by the ventilating system. This again suppresses the desired oxygen delivery effectiveness of the ventilation system. When the patient is under a critical condition, such delivery ineffectiveness and/or inaccuracy may endanger the patient.
Therefore a substantial need exists in the art to provide a gas ID that can automatically and reliably detect the exact kind of gas supplied to a ventilating system, so as to provide an exact oxygen flow rate required by the patient. Further, correction for calibration of the blender and the flow sensors is also needed to appropriately assist the property prescribed respiration therapy of the patient.
The present invention provides an automatic electronic gas identification ID, which preferably comprises a gas inlet and a voltage divider attached on the gas inlet of the ventilator. The gas inlet interlocks a gas source such as a conventional gas bottle or tank with the gas delivery system. When the gas is supplied to the gas delivery system, the voltage divider is inserted, i.e., plugged in to the ventilator. The voltage divider includes a resistor, across which a voltage drop is measured to identify the actual gas composition supplied to the gas delivery system. A lookup table in which a list of voltage drop values corresponding to the various gases used in ventilation therapy is stored is further included. Once the voltage drop is measured, the corresponding gas is determined from the lookup table to provide an automatic fail-safe identification of the actual gas composition supplied to the ventilator.
The present invention further provides a blender applied to a gas delivery system to blend a gas and oxygen supplied therein. The blender is driven by an actuator preferably implemented as a stepper motor for driving the blender to various blending positions according to a desired oxygen flow rate of a mixture of the gas and oxygen. The blender positions are calibrated based on characteristics of the gas, such as the specific heat ratio and the gas constant.
The present invention also provides one or more flow sensors installed in a gas delivery system to detect a flow rate of a mixture of a gas and an oxygen delivered and/or expelled by the gas delivery system. The flow sensor is calibrated according to temperature, humidity, barometer pressure and composition of the gas. When the flow rate of the gas drops under a certain value, the viscous force dominate the flow rate. Therefore, the viscosity of the gas has to be considered as a factor in the calibration of the flow sensor.
The gas delivery system of the present invention includes a ventilating system, which comprises two gas inlets, with one connected to an oxygen source and the other one connected to a gas source, the gas ID, the blender and one flow sensor installed in an inspiratory circuit, a proximal circuit, and an expiratory circuit thereof. A flow control valve may also be installed on the inspiratory circuit to adjust the flow rate according to reading of the inspiratory flow sensor. The inspiratory flow sensor, the proximal flow sensor and the expiratory flow sensor are calibrated based on a reference volumetric flow calculated at a body temperature, a gas constant of air, and a barometric pressure. Such reference volumetric flow is pre-stored in a calibration table.
Accordingly, methods of correcting the calibration of the blender and the flow sensor for gas composition are also provided by the present invention. In the method of correcting calibration of the blender, the oxygen flow rate in air is calibrated as a multiplication factor of a blender position for air. A volumetric flow ratio of a gas supplied to the blender to the air is derived. The multiplication factor for air is substituted with a factor derived from the gas volumetric flow ratio of the gas, a volume fraction of oxygen in the gas, and an oxygen flow rate of the gas. The volumetric flow ratio is determined by gas constants and specific heat ratios for air and the gas.
In the method of correcting the calibration of a flow sensor, a standard volumetric flow for air with 21% of oxygen at an ambient pressure and temperature is derived. The standard volumetric flow for air is then converted into a reference volumetric flow for air under a body temperature and a standard pressure. The standard volumetric flow is then multiplied by a correction factor derived according to actual temperature, humidity, pressure and a gas constant of the gas applied to the flow sensor.
These, as well as other features of the present invention, will become more apparent upon reference to the drawings wherein:
An automatic alarm system may also be utilized in the present invention. As will be recognized, the onboard processor may be configured to allow the clinician inputting the type of gas required by the ventilating system. Once the gas source is connected, the gas ID automatically identifies the gas supply. When the identified result is different from the input gas, an alarm signal or warning indication may be initiated. Alternatively, operation of the system may be halted when a differing gas is connected. Further, an adaptor or a connector may be used to allow the gas ID connecting different gas sources to the system.
Being blended by the blender 43, the gas mixture is the delivered along the inspiratory circuit 32 to the patient. As mentioned above, under different condition or for different patients, the required gas flow rate is different. Therefore, it is crucial to monitor whether the actual delivered gas flow rate is maintained as predetermined. A flow sensor 44 is thus installed along the inspiratory circuit 32. A flow control valve 45 can also be installed to adjust the gas flow when the reading of the flow sensor 44 is different from the desired pre-set value. In the preferred embodiment of the present invention, the flow sensor 44 measures the pressure drop that is directly related to the gas flow rate across an orifice thereof. The pressure drop is transferred into an electronic signal, and the voltage of the electronic signal directly reflects the gas flow rate of the gas mixture. Again, the voltage that reflects the gas flow rate is different depending on the gas type. Thus, the flow sensor 44 must be calibrated according to the exact gas mixture before being used to provide a correct reading. As previously stated, typically, the flow sensor is initially calibrated for air only. Therefore, when a gas mixture other than air flows through the flow sensor, the calibration of the flow sensor 44 has to be corrected based on the characteristics of the particular gas mixture and the actual delivery conditions. Similarly, flow sensors 45 and 46 are also installed at the port of the proximal circuit 36 directly applied to the patient and the expiratory circuit 34 through which the patient exhales to properly monitor the respiration. As with the inspiratory circuit flow sensor 44, the proximal and expiratory flow sensors 45 and 46 typically initially calibrated for air must be recalibrated/corrected based upon the particular gas mixture and respiratory conditions. The preferred correction of calibration of the blender 43 and each of the flow sensors 42, 44 and 46 will be introduced as follows:
In the following paragraphs, correction of calibration for the blender 43 position and the flow sensors 44, 45 and 46 are introduced using heliox mixture as an example. The correction for gas mixture other than heliox such as for nitrogen and/or carbon dioxide can be derived by the same algorithm. For the air containing about 21% of oxygen, the oxygen flow rate of the air mixture (of oxygen and air) can be represented as:
Table 1 shows the corresponding gas constants, specific heat ratios, and volume fractions of the gas, and Table 2 shows the corresponding flow correction factors for various upstream pressures and downstream pressures of heliox with different volume ratios.
Therefore, when the gases supplied to the blender 43 includes a mixture of oxygen and heliox instead of a mixture of oxygen and air, the blender position should be controlled in relation to (FIO2|Heliox) instead of (FIO2|Air), and the blender position as shown in Equation (4) can be modified as:
Based on gas dynamics theory, a general correction factor for the variable orifice flow sensors 32, 34 and 36 is related to gas temperature, relative humidity, barometer pressure (including back pressure) and gas composition. In addition, such variable orifice flow sensor measure the pressure drop across the orifice (ΔP) as a function of volumetric flow. However, under different conditions (that is, when any of the above correction factors is altered), same orifice geometry and ΔP may represent different flow. Therefore, the flow sensors have to be corrected under different conditions before being used for specific respiratory measurements.
For a perfect gas and isentropic gas flow, the mass flow can be expressed as:
When the condition A (TA in ° K or ° R, RA, P2A) results in the same pressure drop across the same flow sensor as the condition B (TB in ° K or ° R, RB, P2B) does, the volumetric flow under conditions A and B have the relationship as:
As mentioned above, the pressure drop across the flow sensor is directly related to the humidity, therefore, the vapor pressure of the gas needs to be calculated for calibration of the flow sensor. A humidifier is typically installed into gas delivery system to provide a certain percentage, preferably 100%, of humidity to the patient. The volume fraction of H2O vapor can be obtained from:
However, the isentropic flow assumption is only valid when viscosity of the gas flow is negligible, which requires a high Reynolds number. For a 0.4 LPM (liter per minute) gas flowing in a one-inch pipe, the high Reynolds number requirement (Re=22) may not be valid. Therefore, Equations (9) and (10) must be validated with experimental results for low gas flow rates.
The volumetric flow ratio of conditions A to B in Equation (10) is equal to (ρB/ρA)1/2 for a perfect gas. That is, the volumetric flow ratio is related to the density of the gas. This is consistent with the isentropic flow assumption in which inertia force dominates. On the other hand, if Reynolds number is low for a very low flow rate in the viscous force dominant condition, the volumetric flow ratio is viscosity μ related and can be expressed as QA/QB=μA/μB according to Poiseulle law. For intermediate flow rates, both inertia and viscous force are present, an empirical equation for flow correction is in a form of:
For variable orifice flow sensor installed along the inspiratory circuit,
In the practical application, the calibration conditions temperature (TC), the gas constant (RC) and the barometer pressure (PC) may be different from the standard calibration condition (ATP). For example, for air mixture flowing through the flow sensor 44 installed along the inspiratory circuit 32, the calibration conditions RC is replaced by TB (which is 54.6078 for air), TC is replaced by TB, which is the body temperature 557.67° R, and PC is replaced by PB, which is 14.696 psia. The volumetric flow VC is thus converted into VB, which is expressed as
By combining Equations (15) and (16), and substituting Pf=PBAR, Ps=PBAR+PINSP and Tf=TC=TB=37° C.,
Therefore, when heliox gas mixture is supplied to the system, the flow sensor is corrected based on the characteristics of the heliox gas mixture. The reading of the flow sensor is FBTPS for heliox mixture instead of Fi for air. That is, the flow rate for air (Fi) should be multiplied by the correction factor in Equation (19) to provide the actual gas flow rate.
Helium is a gas that has a higher viscosity compared to air. Therefore, once the flow rate drops to a certain magnitude, the viscous force dominates. The correction for the flow sensor thus has to be converted from the density dominant format into the viscosity dominant. Such situation is also considered when other gas is applied to the system, however, the critical point for the conversion may be altered for different gas. In this embodiment, heliox mixture is used as an example to introduce the correction of the flow sensor when the viscous force dominates.
When a heliox mixture is applied to the system, the volumetric flow becomes viscosity dominant when the flow rate is lower than 4.06 LPM. Under such circumstance, an empirical relation is obtained as:
Therefore, when the flow rate of heliox mixture is higher than 4.06 LPM, the flow rate is derived by Equation (19). However, when the flow rate drops to lower than 4.06 LPM, Equation (20) is used to calculate the flow rate.
The flow sensors located at the proximal circuit and the expiratory circuit can be calibrated and corrected by the same methods. However, as the calibration conditions of the flow sensors 45 and 46 located at the proximal 36 and the expiratory circuit 34 are normally different from those at the inspiratory circuit, the above parameters have to be adjusted. For the proximal-circuit, Equation (19) is modified as:
As shown from the above equation, the humidity of the proximal circuit 36 is different for inhalation and exhalation processes, therefore, the gas constant is varied. Further, as the proximal temperature is close to the body temperature, such that the factor of square root of TINSP is eliminated by the factor of square root of TB in C2.
For the expiratory circuit 34, the expiratory temperature normally drops from the body temperature to about 5.5° C. lower. This results in a drop of vapor volume fraction in the expiratory circuit. Therefore, with Ps=PBAR+PEXP and the other same conditions,
Again, when the flow rate is lower than a certain magnitude, the viscosity dominates. The flow rate is then related to both the density and the viscosity of the gas. Equation (20) can also be applied to the calibration correction for the proximal circuit 36 and the expiratory circuit 34 flow sensors.
Indeed, each of the features and embodiments described herein can be used by itself, or in combination with one or more of other features and embodiment. Thus, the invention is not limited by the illustrated embodiment but is to be defined by the following claims when read in the broadest reasonable manner to preserve the validity of the claims.